Automatic volume control circuit



Aug# 25, 1942 R. L. HoLLlNGswoR-rH .2,294,117

AUTOMATIC VOLUME CONTROL CIRCUIT Filed June 4, 1940 EE HOLLINGSWORTH BY77% M TTOHNE Y Patented Aug. 25, 1942 ire-D STAT-ss enr-snr ofrFl-f-ceto Radio Corporation o tion of Delaware f America, a v.corpora-Application June 4, 1940, Serial'No.,338,'680

(Cl. .Z50-20) Claims.

My present invention relates generally to automatic gain controlcircuits, and more particularly to automatic volume control circuits ofthe vtype adapted for use in receiving signals in the ultra-short waveband.

Automatic gain control arrangements which are in use at the present timein receiving sig- `nals in the 4-30 megacycle (mc.) band have severaloperating limitations. In the first place, the rate of vcompensation isthe same for ascending and descending signal carrier levels with theresult that on the ascending portions of the fading periods blastingeffects are had. In other words, during the rapid fading which isexperienced in the ultra-short wave band, the conventional type ofautomatic gain control circuit `produces a rate of compensation which isnot wherein the action of the control circuit approaches that which ishad in short wave communication when employing two-set diversityreception for telephone communication, and the essential and outstandingcharacteristics of the novel control circuit comprising instantaneousfading compensation without reducing the low audio frequency response,and the AGC time constant network being so chosen as to cause smoothreceiver gain control action thereby substantially eliminating blastingeffects on the ascending portion of the rapid fade.

Another important object of this invention is to provide an automaticvolume control circuit for a short wave receiver wherein the volumecontrol circuit is provided with a device of unidirectionalconductivity, and a time constant network whose magnitude issufficiently large to provide substantially smooth fadingcompensationffor rapid fading without effect on the lower audiofrequencies of the modulation range.

Still other objects of my present invention are to improve generally theeiiciency and reliability of automatic gain control circuits for shortwave receivers, and more especially to provide an automatic gain controlcircuit for short Wave receivers which is simply and economically`assembled inthe receiver.

The novel features-Which I believelto be characteristic of my inventionare set forth in :particularity in theappendedclaims; the vinventionitself, however, `as to both -its organization and method ofoperationwill best be understood :by reference to the followingdescription taken in connection with the drawingin which Ihave indicateddiagrammatically Va circuit organization whereby my invention maybe-carried into effect.

In the drawing:

Fig. 1 shows a short wave superheterodyne receiver embodying the presentinvention,

Fig. 2 graphically shows the effect of the invention on the lowerportion of the audio range,

Fig. 3 graphically illustrates the manner in which the present inventionprovides smooth fading compensation.

Referring now to the receiving system shown in Fig. 1, it is to beunderstood that the latter may be a superheterodyne receiver of the typeshown employed in short Wave telephony. The receiving band in such areceiver may be of the order of 4-30 mc., and the receiver will beprovided with a dipole I* for collecting the short wave signals. Theantenna may be of the usual grounded antenna type if desired, or it mayeven be a radio frequency line capable of distributing signals in thespecied signal range. The signal collector device is coupled to thetunable input circuit 2 of the radio frequency amplifier 3, and theamplier may comprise one or more stages of tunable modulated carrieramplification. The amplified modulated carrier signals are impressedupon the tunable input circuit 4 of a following rst detector 5, and theoutput circuit of the latter is designated by numeral 6.

It will be understood that circuit 6 is resonated to the operatingintermediate frequency (I. F.). The latter may be chosen from a range of'l5-450 kc. The numeral 1 denotes the usual tunable local oscillatorwhich produces oscillations of a frequency constantly diiering from thefrequency'of the signals impressed'upon the rst detector 5 by the valueof the I. F. The numeral 8 designates the customary Yand conventionaltuning control mechanism which mechanically couples the adjustableelements ofthe various tuning devices of the tunable circuits ofnetworks 3, 5 and 1. The I. F. amplifier 9 may comprise one or morestages of I. F. amplication, and the input circuit I3, coupled to thedetector ,output circuit 6, is Xedly-tunedito the I. F. value.

The I. F. output circuit II is coupled to the input circuit I2 of thesecond detector I3. The latter is shown as being of the diode type, andhas its anode connected to the high alternating potential side of theinput circuit I2. The low potential side of input circuit I2 isconnected to ground by a load resistor I4, while the cathode of diode I3is also grounded. There is developed across resistor I4 rectied voltagewhich is derived from the modulated carrier signals at I. F. The audiofrequency component of the rectified I. F. currents is fed to asucceeding audio frequency network through condenser I5. 'Ihe audionetwork may comprise one or more stages of audio amplication, and thelast stage may be followed by any desired type of reproducer. The directcurrent voltage component of the rectified I, F. currents is utilized intwo paths now to be described.

One of these paths comprises a so-called noise silencer device. Thedevice comprises an electron discharge tube I 6 whose input grid I'I isconnected to the anode end of resistor I4. The plate I8 of tube I6 isconnected to ground through a path which includes the direct currentsource I9 and the resistor 20, it being noted that the plate I8 isconnected to the positive terminal of source I9. The cathode of tube I6is at ground potential. Hence, when normal electron iiow takes placefrom the cathode to the plate I8 of tube I6 direct current voltage isdeveloped across resistor 20.

The magnitude of the direct current voltage developed across resistor2|] depends upon the bias of grid I'I with respect to the cathode oftube I 6. The bias of grid I I in turn is dependent upon the directcurrent voltage developed across resistor I4 due to rectication of theI. F. signal currents. The second path which utilizes the direct currentvoltage developed across resistor I4 comprises a device ofuni-directional conductivity, such as diode 2I. The cathode 22 of thediode 2I is connected through resistor 23 to the anode end of resistorI4. The cathode 22 is also connected through resistor 25 to theungrounded end of resistor 2i). The anode 30 of diode 2I is connected toground through the resistor element 3l, the latter being shunted bycondenser 32. The anode 30 has direct current voltage connections 40 and4I to the stages of the receiver whose gain is to be controlled independence on the magnitude of the signal carrier. In other words, theconnections 40 and 4I are the automatic volume control (AVC), orautomatic gain control, which is utilized to vary the gain of each ofthe controlled stages in such a sense that the carrier amplitude at thedetector input circuit I2 is substantially uniform over a wide range ofcarrier amplitude variation at the signal collector I.

In the absence of received signals there is substantially no directcurrent voltage developed across resistor I4. Consequently, the grid I1of tube I6 has minimum negative bias. This results in maximum spacecurrent now through tube IB with a resulting large direct currentVoltage developed across resistor 20. Since cathode 22 of diode 2| isconnected to the end of the resistor 20 which becomes increasinglynegative with respect to ground as the voltage drop across resistor 20increases, it follows that anode 30 of diode 2I will assume a positivepotential with respect to cathode 22. In that case diode 2| isconductive, and, therefore, the controlled stages 3 and 9 will havenegative bias applied thereto.

In other words, assuming that the signal grids of each of the radiofrequency and intermediate frequency amplifiers are connected toconnections 40 and 4I, then these signal grids will be established at ahigh negative bias when the drop across resistor 20 is a maximum. As aresult the gain of controlled stages 3 and 9 will be greatly i reduced,and the electrical noise impulses, which audio frequencies.

normally would be detected at I3, will not be heard due to the greatlyreduced gain of ampliers 3 and 9.

However, as the received signals increase in amplitude, direct currentvoltage is developed across resistor I4. This causes the negative biason grid I1 .to increase thereby reducing the voltage developed acrossresistor 20. A point is soon reached where the diode 2| isnon-conductive. This means that the gain of each of stages 3 and 9 isrestored to optimum. As the received signals increase in amplitude abovea predetermined weak carrier amplitude the direct current voltagedeveloped across resistor I4 increases in magnitude, and eventuallycompletely cuts off tube I6 so that no voltage is developed acrossresistor 20. However, since the cathode 22 is connected through path 23to the anode end of resistor I4, the cathode 22 becomes negative withrespect to the diode anode 30 with the result that the diode 2| againbecomes conductive, and permits the increasing negative bias derivedfrom resistor I4 to be applied over connections 40 and 4I as an AVCbias. Resistors 23 and 25 function as filter resistors, and may eachhave magnitudes of approximately 200,000 ohms.

The network 3I-32 has a large time constant, and resistor 3I may have avalue of approximately 1 megohm whereas condenser 32 may be chosen froma range 0.1 to 1.0 microfarad. The large time constant network 32-3Ipermits very fast AVC action to occur without reducing the low As amatter of fact there is had an accentuation of the lower audiofrequencies. In Fig. 2 there is shown (curve A) the reduction of thelower audio frequencies, up to about 100 cycles, when the time constantnetwork 32-3I has a small time constant value. The curve B denotes theaccentuation of the lower audio frequencies up to about 100 cycles,caused by the utilization of the large time constant network inconjunction with the diode device 2|. This compensation of low audiofrequency curtailment, and indeed accentuation of the lower audiofrequencies, is of advantage in receiving short Wave telephony signalssince the quality of these received signals is maintained substantiallyhigher than has been thought hitherto possible.

There is also secured smooth control action because of the provision ofa reasonable amount of hold over in the time constant network 32-3I. InFig. 3 the full line curve shows an illustrative rapid fading period.The dash line surve, designated Prior art AVC, illustrates the type offading compensation which has been secured in the prior art when using afast acting AVC circuit. The dash line curve designated Effect of largetime constant illustrates the smooth AVC action which is secured withthe present invention, and such smooth rapid fading compensation isprovided by the combined actions of the diode 2I and the large timeconstant network 32-3I. Automatic gain control systems in use at thepresent time have a rate of compensation which is the same for ascendingor descending signal carrier levels.

On the ascending part of a fade, the present invention permits thenumber of negative electrons to increase very rapidly in the timeconstant network 32-3l through a one-Way circuit thereby positivelycontrolling the upper halves of the carrier magnitude increase. If aconventional gain control system were allowed to operate as fast as inthe present case, reduction of the audio frequency compensation wouldresult. In the present case, as has been previously pointed out, themagnitudes of the time constant elements are increased so that there isproduced a considerable amount of audio compression in the receiveraudio output. The instantaneous action of the AVC circuit throughunidirectional device 2| permits the condenser 32 to act in the mannerof a low audio response equalizer across the diode output, but onlythrough that period of time when the flow of electrons will be increasedthrough the series feed tube, i. e., on sudden carrier level increasesor on modulation level increases. A degree of compression with improvedcarrier control greatly improves the overall equality of the set whenlistening in on short wave,'

long distance circuits.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular` organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What I claim is:

l. In an automatic volume control circuit for a radio receiver, arectifier having a signal input circuit and a load element across whichis developed rectified signal voltage, and automatic volume control pathconnected to said load element which includes a series device ofuni-directional conductivity, said device being rendered conductive upondevelopment across said load element of rectified voltage whosemagnitude eX- ceeds a predetermined amplitude, and a relatively largetime constant network operatively associated with said series device forrendering the automatic volume control fast and relatively smooth, asecond device having a second load element in circuit therewith fordeveloping a direct current voltage, means for applying said directcurrent voltage to said rst device thereby rendering the latterconductive for signals less than a predetermined minimum amplitude, andmeans responsive to the voltage developed across said rst load elementfor rendering the second device ineffective.

2. In a radio receiver provided with -a detector adapted to developdirect current voltage whose magnitude is proportional to the receivedsignal carrier amplitude, an automatic gain control connection to saiddetector, said connection including a diode in series therein, saiddiode being rendered conductive upon the development of said voltage toa value above a predetermined magnitude, means responsive to saidvoltage when less than said predetermined magnitude for rendering saiddiode conductive independently of said developed voltage, a timeconstant network of relatively large magnitude operatively associatedwith said diode.

3. In a radio receiver of the type including a modulated carrier signalamplifier arranged in cascade with a detector, means operativelyassociated with said detector for deriving a uni-directional voltagefrom detected modulated carrier signals, an automatic gain controlconnection between said detector and said amplifier, said connectionincluding a device of uni-directional conductivity which is soconstructed, and so arranged in said connection, that the device becomesconductive in response to said uni-directional voltage exceeding `apredetermined amplitude, means responsive to said voltage decreasingbelow said amplitude for rendering said device conductive thereby tobias said ampliiier, and said connection including a large time constantnetwork for rendering the volume control action relatively fast butrelatively smooth.

4. In a radio receiver of the type including a modulated carrier signalamplifier arranged in cascade with a detector, means operativelyassociated with said detector for deriving a uni-directional voltagefrom detected modulated carrier signals, an automatic gain controlconnection between said detector and said amplifier, said connectionincluding a device of uni-directional conductivity as the sole couplingelement between the detector and amplifier, said device being soconstructed, and so arranged in said connection, that the device becomesconductive in response to said uni-directional voltage exceeding apredetermined amplitude, means responsive to Said uni-directionalvoltage in excess of said predetermined amplitude for rendering saiddevice conductive, said last means being rendered inef- `fective inresponse to an increase in uni-directional voltage substantially abovesaid predetermined amplitude, and said connection including a large timeconstant network for rendering the volume control action relatively fastbut relatively smooth, and the constants of said time constant networkbeing so chosen that the low modulation frequency amplitudes aresubstantially accentuated.

5. In a radio receiver of the type including a modulated carrier signalamplifier arranged in cascade with a detector, means operativelyassociated with said detector for deriving a unidirectional voltage fromdetected modulated carrier signals, an automatic gain control connectionbetween said detector and said amplifier, said connection including adevice of uni-directional conductivity which is so constructed, and soarranged in said connection, that the device becomes conductive inresponse to said uni-directional voltage exceeding a predeterminedamplitude, and said connection including a large time constant networkfor rendering the Volume control action relatively fast but relativelysmooth, a device adapted to produce a second uni-directional voltage,means for applying said second voltage to said device of uni-directionalconductivity thereby rendering the latter conductive when the secondvoltage has an amplitude above a predetermined value, and meansresponsive to said first uni-directional voltage for rendering the saidsecond uni-directional voltage device ineffective.

R, LEE HOLLINGSWORTH.

